A known way of performing modulation of the supervisory signal on WDM channels is by using Raman gain. However as Raman amplification is not flat over C band (1520-1570 nm) or L band (1570-1610 nm), it is difficult to achieve uniform modulation over all WDM channels.
When Raman gain is used, a relatively high power pump laser emitting at a wavelength of about 1455 nm, two 1455/1550 WDM couplers and about 4 km of NDSF (Non-Dispersion-Shifted Fiber) are needed, making the supervisory modulator expensive, bulky and lossy with insertion loss in the range of for example 3 to 4 dB.
On the other hand, in cases where a WDM branching unit is present in the submerged plant, optical Erbium-doped fiber amplifiers (EDFA) are conventionally used for overcoming losses in the branching unit caused by passive optical components and for balancing the relative optical power values between the straight-through channels and the channels added in the trunk line. Furthermore, optical amplifiers provide the capability of supervisory return signaling necessary for sending management information from the branching unit to a terminal station.
However, optical amplifiers add cost, noise, complexity and volume, while worsening the reliability of the operation and creating manufacturability problems for the branching unit.
It is therefore desired to obtain an efficient and cost effective scheme of supervisory modulation without the need for using Raman modulation or optical amplifiers thus avoiding the drawbacks mentioned above.